1. Field of Inventions
The present invention relates generally to a method and apparatus for controlling firing energy in a printer and, more specifically, to a method and apparatus for non-saturated switching for firing energy control in an inkjet printer.
2. Description of the Related Art
Thermal inkjet printers employ nozzle resistors to fire drops of ink. A sufficient amount of energy must be provided to each nozzle resistor to properly fire the drops of ink. If an amount of energy delivered to a nozzle resistor is too low, there may not be enough heat generated to eject an ink drop, or the velocity of the drop may be too low. Either condition may result in visible defects in the printed page. If the amount of energy delivered to a nozzle resistor is too high, the resistor may get too hot resulting in decreased pen life. For these reasons, accurate energy control is essential for proper operation of thermal inkjet pens.
Referring to FIG. 1, a control electronics/ inkjet pen system 100 of an inkjet printer includes a main electronics board 102, an inkjet pen 104, an interconnecting cable 106 and associated connectors 108, 110 at each end of the cable 106. An exemplary preferred electronics board 102 includes a voltage regulator circuit 112 for creating an accurate pen voltage and a pen driver integrated circuit (IC) 114 containing solid state switches for turning nozzle currents on and off.
When the driver switches are turned on, electrical current flows from the pen voltage supply at board 102, through the cable 109, through the nozzle resistors in the pen 104, and returns back through the cable 106 to the ground side of the pen voltage supply. Since none of these components are ideal, there are losses associated with each of them. For instance, switches of the pen driver IC 114 have some resistance that creates a voltage drop when current flows through them. Likewise, the cable 106 and connectors 108, 110 have resistances of their own resulting in further losses. Since these resistances are not exactly known and vary from printer to printer and over temperature, the amount of current flowing through the nozzle resistors is difficult to perfectly control. Other contributors to energy errors stem from the tolerance of the generated pen supply voltage and variations in the resistances of the nozzle resistors themselves.
FIG. 2 shows an electrical schematic representation of the system of FIG. 1 including non-ideal parameters which contribute to errors in delivered energy. In this schematic, VSupply represents the voltage of the pen voltage supply, RSeries represents the series combination of the cable and connector resistances, TFire is the time for which the switch is closed, and VSwitch is the voltage drop across the switch when current is flowing while the switch is closed. Energy variations due to the loss across the switch contribute significantly to the energy error and, for the electrical schematic of FIG. 2, are calculated as follows:       E    Fire    =                    (                                            V              supply                        -                          V              switch                                                          R              Series                        +                          R              Pen                                      )            2        xc3x97          R      Pen        xc3x97          T      Fire      
In this equation, the current flowing through RPen is given by the term in parentheses, which is equivalent to the voltage across both resistances divided by the sum of the resistances. Since the energy is proportional to the square of the current, the energy will change at approximately twice the rate the current changes. In other words, if the current is allowed to vary by xc2x11%, the energy will vary by xc2x12%. If the current varies by xc2x15%, the energy will vary by xc2x110%, etc. This is a result of the fact that a change in something is equivalent to its derivative, and the derivative of x2 (with respect to x) is 2.
Since the term inside the parentheses is equal to current, the current is proportional to the quantity (VSupplyxe2x88x92VSwitch) As this quantity changes, the energy delivered to the pen changes at twice the rate. Assuming the supply voltage is known exactly, it is possible to determine how variations in the switch voltage affect the delivered energy. Since the supply voltage is greater than the switch voltage, a variation in the switch voltage will result in a smaller variation in the overall quantity (VSupplyxe2x88x92VSwitch). Thus, variation in current is determined by the following equation.
Variation in current=xcex94I=xcex94(VSupplyxe2x88x92VSwitch)=xcex94VSwitch*(VSwitch/VSupplyxe2x88x92VSwitch))xe2x80x83xe2x80x83Eq. 1
where xe2x80x9cxcex94xe2x80x9d indicates a percent variation in the corresponding value. For instance, if VSupply is five times greater than VSwitch, VSwitch/(VSupplyxe2x88x92VSwitch) would be 0.25, and variations in VSwitch would result in one fourth the variation in current. By way of example, where VSupply is 12.0 volts and VSwitch is 1.3 volts xc2x130%:
xe2x80x83Variation in current=xcex94I=30%*(1.3/(12.0xe2x88x921.3))=3.6%.
Recall that variation (or tolerance) in the energy delivered to the pen is twice the variation in current since energy is proportional to the current squared. Therefore, the energy tolerance due to the switch voltage tolerance is doubled to 7.2%. By itself, this is already in violation of the specified limits for some inkjet pens. An understanding of each of the parameters in the electrical schematic of FIG. 2 would be useful to the end of tightening all of the tolerances as much as possible. With respect to the switches in the pen driver IC 114 (FIG. 1), it would be useful to be able to accurately characterize the voltage drop across the switches for improving the accuracy in delivered energy.
Past architectures have attempted to solve this problem by making the switch voltage drop as small as possible. In practice, these switches are transistors (field-effect or bipolar) that are designed to have very low resistance and voltage when they are turned on. By making this voltage very small, the overall error contributed by the switch voltage drop is less (see Equation 1). However, implementing such very low on-resistance transistors in an integrated circuit requires that the transistors occupy a relatively large area of the silicon die. When many of these transistors are contained on the same die (which is usually the case with typical pen driver ICs), the area of the die can become fairly large, resulting in increased cost for the IC. For instance, to reduce the on-resistance between the drain and source (RDSon) of a field effect transistor, many small transistors are connected in parallel to form a compound transistor such that the overall channel resistance reduction is proportional to the number of individual transistors used. The RDSon of these transistors in typical pen drivers is kept small enough that, when current passes through the switch, the voltage drop is small enough to yield an acceptable variation in energy. Notwithstanding, there remains a need for a method and apparatus for firing energy control in a printer that maintains an acceptable tolerance for the voltage drop across the driver transistors to precisely control the amount of energy provided to the nozzle resistors while keeping the size of the driver transistors relatively small.
According to the present invention, a method and apparatus for controlling firing energy in an inkjet printer reduces energy errors induced by the voltage drop across the switch by first accurately characterizing this voltage drop. Since the voltage drop across the switch is well characterized, the pen voltage can be increased to compensate for this loss (i.e. (VSupplyxe2x88x92VSwitch) is kept constant by increasing the supply voltage by an amount equal to the switch voltage drop). The firing energy control implementation of the present invention keeps the voltage across the pen and current well characterized; and the energy delivered to the pen is therefore controlled more accurately. Additionally, the firing energy control implementation of the present invention facilitates the employment of a driver IC with smaller driver transistors which results in space and cost savings in the driver IC.
The present invention exploits the fact that, for accurate energy control, the voltage drop needs to be well characterized, but does not necessarily need to be small. Even if the voltage drop across the switch is large, if the tolerance of the voltage drop is tight, the contributed energy fluctuations may still be kept small by employing the pen voltage supply to compensate for this known voltage drop across the switch. In an exemplary preferred embodiment, this is accomplished by operating the switching transistors just outside the saturation region and using a voltage monitor to control the switch voltage drop.
A method for controlling firing energy in an inkjet printer in accordance with one embodiment of the present invention includes the steps of: controlling a voltage across a low side driver which is electrically connected to a nozzle resistor of an inkjet printer pen; and adjusting a pen supply voltage which is electrically connected to the pen to compensate for changes in the voltage across the low side driver.
A method for controlling firing energy in an inkjet printer in accordance with another embodiment of the present invention includes the steps of: controlling a switch voltage across a switch which is electrically connected to a nozzle resistor of a printer pen; and adjusting a pen supply voltage which is electrically connected across the pen and the nozzle resistor to compensate for changes in the switch voltage.
An apparatus for controlling firing energy in an inkjet printer in accordance with another embodiment of the present invention includes: an inkjet pen including a nozzle resistor; a control circuit including a switch electrically connected between the nozzle resistor and a low voltage rail, the control circuit being configured to control a switch voltage across the switch; and a regulated pen voltage source which provides a pen voltage to the nozzle resistor, the pen voltage being adjusted to compensate for the voltage drop across the switch.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.